The government has been forced into a major U-turn on the NHS by introducing a new waiting time target in order to tackle the growing number of patients not undergoing surgery within the promised 18 weeks.

The health secretary, Andrew Lansley, had previously criticised waiting times introduced by Labour in order to speed up patient care as "arbitrary Whitehall targets".

But fresh evidence that waiting times are creeping up, despite David Cameron's pledge to keep them low, emerged on Thursday, forcing Lansley to change tack and impose an additional treatment standard on the NHS.

The move is a surprise because the health secretary had previously castigated targets as unnecessary, likely to distort NHS staff's clinical priorities and part of a bureaucratic "top-down" system he intended to overhaul.

It has been prompted by the disclosure that around 242,000 patients do not get treated within the 18 weeks guaranteed in the NHS constitution.

That represents 9.4% of the 3 million people awaiting treatment at any one time, and about 20,000 patients have been left untreated for at least a year.

On Thursday, Lansley warned the NHS in England that, as of next year, no more than 8% of all patients waiting at any one time would be allowed to have had their treatment delayed by 18 weeks or more.

The service's progress towards meeting this target will be monitored closely, he said, as he belatedly embraced the sort of tactics Labour used to cut the long waiting lists it had to confront early in Tony Blair's premiership.

Lansley's decision came after he had insisted for months, despite evidence to the contrary, that NHS waiting times remained "low and stable".

But Department of Health data shows the number of people having to wait for more than 18 weeks has been steadily increasing since the coalition came to power in May 2010.

Labour were quick to seize on Lansley's new approach, saying the government's new policy on NHS waiting times was "an admission of failure" and proof that the health secretary had refused to heed warnings that relaxing waiting-time targets would inevitably mean unnecessary pain, discomfort and frustration for patients.

"One of his first acts in office was to relax Labour's waiting-time standards," he said. "We warned him that patients would pay the price, and this is exactly what has happened.

"It's because of Andrew Lansley's failure to get a grip on waiting times that he's being forced to bring out these new rules. He would do better instead to focus on bringing waiting times back down to the historic low that he inherited from Labour."

The deteriorating situation would worsen if ministers pressed ahead with another controversial Lansley policy – that of abolishing the cap on the amount of income semi-independent foundation trust hospitals can make by treating private patients.

Critics fear it will lead to the well-off being prioritised and ordinary NHS users having to wait even longer because staff and operating theatres are being used to treat fee-paying patients.

"Sadly, things will get even worse if he succeeds in abolishing Labour's cap on the amount of private work hospitals can do. This will take us straight back to the bad old days of the Tory NHS, where patients are forced to choose between waiting longer or paying to go private," Burnham said.

The official data showed that 17,873 more people a month are waiting longer than in May 2010, the month the coalition took office and relaxed NHS waiting time targets soon afterwards.

When comparing September 2011 with September 2010,9,967 people had waited longer than the supposedly maximum waiting time the government had guaranteed. Those comparisons include inpatients and outpatients and all those treated at the NHS's expense, either in an NHS hospital or by a private healthcare provider.

A total of 20,662 (7.1%) of inpatients treated in May 2010 had waited more than 18 weeks, but that figure rose to 23,542 (7.5%) in September 2010 and was up again to 28,926 (9.3%) by September 2011 – a jump of 40% over the coalition's first 16 months in office.

Similarly, in May 2010, a total of 15,557 outpatients (1.8%) had to wait beyond the 18 weeks, rising to 20,583 (2.2%) in September 2010 and again to 25,116 (2.8%) by September this year – a rise of 65% over the same period.

In total, 54,092 people receiving treatment in September paid for by the NHS, in an NHS or private hospital, had to wait more than 18 weeks for their procedure.

The worst affected types of medical treatment for inpatients are trauma – this September, 16% of all such patients had waited more than 18 weeks compared with 13% in September 2010; neurosurgery (15%, up from 10% in May 2010) and ear, nose and throat procedures (11%, up from 8% in May 2010).